Field of the Invention
The invention relates to the preparation of polyamides from aqueous solutions of polyamide-forming reactants, where the preparation of the polyamide takes place at a location other than the location of the preparation of the polyamide-forming reactants.
In the formation of polyamides, and particularly those that are useful in production of films and/or fibers, a high molecular weight polymer is normally obtained by polycondensation of low molecular weight polymer that has been prepared from an aqueous solution of polyamide-forming reactants. The low molecular weight polymer is typically produced by a process in which the aqueous solution is first concentrated by evaporating water from the solution and the concentrated solution is then heated to a temperature sufficient to form low molecular weight polymer from the reactants.
In general, the aqueous solution of polyamide-forming reactants includes at least one organic dicarboxylic acid which preferably contains from four to twelve carbon atoms, for example an alkylene or aromatic dicarboxylic acid such as adipic, succinic, sebacic, terephthalic, isophthalic or naphthalic acid, and at least one organic diamine which preferably contains from four to twelve carbon atoms, for example an alkylene or aromatic diamine such as hexamethylene, tetramethylene, pentamethylene, heptamethylene, decamethylene, phenylene or naphthalene diamine. In the preparation of film and/or fiber-forming polyamides, these reactants are generally combined in proportions that are approximately stoichiometrically equivalent and dissolved in water to form a solution containing from about 25% to about 55% and preferably from 40% to 50% by weight of the reactants. Further treatment of such solutions, for example, by pH adjustment, inclusion of additives or purification, e.g., by an absorbent such as activated carbon, may be carried out before polymerization, if desired, to improve the properties of the ultimate polymeric product.
Most desirably, the aqueous solution of polyamide-forming reactants is first heated to evaporate water and thereby concentrate the solution to at least about 60% by weight of the reactants, for example at a temperature between about 105.degree. and about 300.degree. C., and a pressure between about one atmosphere and about 100 pounds per square inch gauge.
Typical of the dicarboxylic acids used in the preparation of polyamides is adipic acid. A well known commercial method of producing adipic acid, a valuable and widely used chemical, involves a series of steps including:
(1) the oxidation of cyclohexane in a liquid phase with air or other molecular oxygen-containing gas to a mixture of cyclohexanol and cyclohexanone at rather low conversion but with high yields;
(2) the separation of the unoxidized cyclohexane from the mixed cyclohexanol and cyclohexanone intermediate reaction product;
(3) the final oxidation of the intermediate product with a strong oxidizing acid, such as nitric acid, into adipic acid and concomitant minor amounts of other dibasic organic acids, including glutaric acid and succinic acid; and
(4) the isolation of the adipic acid from these by-product organic acids, such as by crystallization.
When the adipic acid is separated from the product stream the adipic acid crystals are ordinarily wet from a water wash.
Commercially, it has been a common practice, wherever the preparation of the polyamide forming reactants is at a location other than the location of the formation of the nylon polymer, to dry and then ship the adipic acid. Since the polyamide-forming reactants are ordinarily dissolved in water and reacted therein to form a salt solution, it would be much more desirable to ship the adipic acid wet thereby to eliminate the drying step, such an improvement being an object of this invention. As used herein the term "wet" means water and/or solvent laden.